The present invention relates to nacreous pigments which have improved weathering stability and are based on mica flakes coated with metal oxides and also provided with a stabilizing, chromium-containing second coating.
It is known that plastics or lacquers into which nacreous pigments are incorporated undergo change as a result of weathering. In particular, the polymer matrix into which the pigments are embedded is also destroyed by the combined effect of UV radiation and moisture as a result of the catalytic influence of the mica platelets, which are usually coated with TiO.sub.2.
Stability-improving coatings for pure titanium dioxide pigments are described in the literature, for example, in the book "Titanium," by J. Barksdale, Ronald-Press 1966. However, the coatings described therein are not sufficient to impart adequate stability to the TiO.sub.2 -containing coatings on mica which have a more complicated structure.
A second coating with chromium compounds has been proposed to improve the stability of mica pigments coated with TiO.sub.2. Thus, mica pigments coated with TiO.sub.2 have been additionally coated with methacrylatochromium chloride and exhibit an improved resistance to weathering (German Offenlegungsschrift No. 2,215,191). Mica pigments coated with TiO.sub.2 have also been improved in respect of their resistance to weathering by an additional coating with chromium hydroxide (German Offenlegungsschrift No. 2,852,585).
However, the pigments after-treated in this manner have the disadvantage that the amounts of chromium compounds necessary to achieve an adequate effect have a severe adverse influence on the luster and color quality of the pigments because of their deep gray-green or green intrinsic color.